.22 cap on Impedance Selector Marshall/Splawn??

[Ethan w]

Hi Guys, I have a Marshall 1959SLP modded by Splawn to be 2 channel, much like a Quickrod. I noticed there's a .22 film cap on the impedance selector soldered on the connection for the purple feedback wire and a black wire that goes under the board. It's not the ground connection, must be to the output tranny. Anyone have any idea what this does? Can't say I've seen this in any other amp.

Thx, Ethan

[The Dude]

Hard to say without knowing where the black wire goes, but I'm thinking possibly NFB.

[Ethan w]

I'm thinking the black wire is one of the 3 output secondaries, there's 3 wires for 4, 8 and 16 ohms plus the purple neg feedback wire and another black to ground (on center pin). So assuming that's it, why would you have a .22 cap across the feedback wire and output? That's the question.

[pdf64]

Might the cap be in series with the feedback, thereby giving a bit of bass boost, resonance style?
But 0.22uF seems to high a value to do much

[The Dude]

I was thinking similar (in series- in the NFB), but to tame the high end.

[Ethan w]

I was thinking similar (in series- in the NFB), but to tame the high end.

Thanks for the replies. It seemed too big of a value for a resonance boost, I've made these with .0047 caps. A high end tamer is the best idea so far. I suppose I could ask Scott Splawn. Or I could disconnect it and see what happens. It's not on any Splawn (or Marshall) schems that I can find.

[g1]

Is this a re-ish or original?
I'm not sure if you are maybe misunderstanding (or mis-explaining ) the impedance switch. The center pin can't go to ground, it goes to the hot terminal of output jacks.
If the pin the purple connects to is not also connected to a tap, then it has to get the NFB from somewhere, and this seems to be via the cap.

Edit: or is that lug internally connected to one of the other lugs in the switch?

[Ethan w]

Is this a re-ish or original?
I'm not sure if you are maybe misunderstanding (or mis-explaining ) the impedance switch. The center pin can't go to ground, it goes to the hot terminal of output jacks.
If the pin the purple connects to is not also connected to a tap, then it has to get the NFB from somewhere, and this seems to be via the cap.

Edit: or is that lug internally connected to one of the other lugs in the switch?

You are correct on the center pin, it goes to the hot. The cap is connected between the feedback wire and a black wire that goes under the board. The black wire must be one of the ohm taps from the OT. The lug the purple wire is on does not seem to connect to anything, or I can't find it with my meter anyway. I did a continuity test on that lug and every other lug and got nothing. The purple wire goes to R21 on my amp, which is the feedback resistor. Supposed to be a 47k, it is now a 220K carbon comp via Splawn. The other side of R21 goes to the pres control so I'm sure it's the feedback resistor. Maybe the big cap and increased 220K resistor are some kind of mod by Splawn, fixed resonance or something?

Amp is a 2003 1959SLP reissue, nice amp. Thanks for the schem, I had most of that but not the nice preamp schem, just the earlier hard to read version.


Here's a closer pic:

[g1]

I can't see from the pics, but assume the bottom of the cap & blk wire are connected to a switch lug. That is the 4 ohm tap.
From what I can tell from the schematic, he's moved the nfb from the 8 ohm to the 4 ohm tap, and added the cap in series.
Your component designations (R21) seem to match the schematic from pg.2, not pg.4, but the switch wiring is shown better on pg.4

[aiyiadam]

The cap is possibly there to block the DC and prevent the scratchy noises at the presence pot.

[Ethan w]

The cap is possibly there to block the DC and prevent the scratchy noises at the presence pot.

Thanks for the reply, that makes a lot of sense. The feedback resistor was changed from 47k (?) to 220k so maybe that made the pot scratchy without the big cap.

[aiyiadam]

Thanks for the reply, that makes a lot of sense. The feedback resistor was changed from 47k (?) to 220k so maybe that made the pot scratchy without the big cap.

You are welcome. I have just checked a 50W quickrod schematic, which I had from the slocloneforums. The quickrod seems to have a 220K on the 4ohm tap and a 680nF cap on the presence pot. (And a different PI with 100K grid resistors and 220nF caps). This is very low negative feedback and without the large 680nF presence cap the pot would possibly have very little effect.

On the schematic there aren't any caps on the feedback wire. Maybe splawn wanted to eliminate the possible noises on the presence pot an make an even better amplifier. Personally I can't hear the effect of such larger caps in this position.